When listening with headphones and no speakers connected to the output terminals, the receiver plays fine. When speakers are connected one channel distorts slightly through the speaker and headphone. Looking at the schematic, do you think the 1000uf C13 or C14 near the outputs could be the problem or??

I would doubt it tbh but they are easy enough to prove either by swapping or just bridging another cap across. You never know

If not then check the DC voltage across those 0.5 ohm emitter resistors and compare one channel to the other. I would guess you would see somewhere in the 20 to 30 millivolt region.

Another possibility and just as likely as any real component fault is a poor/tarnished/dirty contact somewhere in the speaker/headphone switching circuitry. The miniscule current drawn by h/phones would pass but the higher speaker current could distort due to the non linear behaviour of a poor connection. Any speaker fuses/fuse holders. They would be suspect too.

I agree, most likely a dirty switch contact - could be at the headphone socket since this is where the amplifier detects the presence of the headphone. Otherwise could be bad output relay on the speaker terminal as it should be an open relay when the headphone is inserted.

By the way, I had a different unit with different amplifiers, but the surprise for me was that the service manual calls for only 2mV across the emitter resistors. Yes, 2mV not 20mV to 30mV. At such low voltage distortion is expected when the amplifier is running at low volume and driving a low impedance such as a speaker. If you are brave you could tweak up the bias on the two channels that feed the microphone output. The heatsink will run warmer.

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Just checked out a cheap little Pioneer SX-3400, the manual calls for anything between 2mv and 250mv idle current!!!! If more than 250mv check for bad parts!!!!! I guess they weren't too concerned on that model.

The headphones don't load the output section like speakers do and you probably aren't using much power when using the headphones. What test equipment do you have? With a signal generator, scope, and load resistor it will take about 2 seconds to see the problem and a little longer to figure what to do about it.

.....the problem only occurs when a speaker is connected to either A or B output. Can not be any switches.

Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!

Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.

Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.

I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.

1000uF caps for speaker coupling is a bit on the low side. Bass performance will be a bit compromised. Up it to at least 2200uF or up to a max of 4700uF. Don't go any further as your turnon thump will get worse, and you risk blowing the 2A fuses, or worse. Use low ESR and 105* types.
Henry

....ppl this >35 years amplifier expect a gozillion problems but all of them easy to fix

---replace all capacitors no questions asked
---soldering here and there
---clean all pots and switches
---verify trimmers are clean enough conductive and operative
---set midpoint and bias
done for another 30 years

As about upgrades UK's suggestion for even triple the capacitance of the output applies , together with similar changes in the power supply primary and secondary , where you might run in to diode size trouble and a bit bigger and faster will be on the safe side , DC blocking capacitors here and there might be replaced with MKT-MKS and that will improve some of the quality, Bypass is a mighty thing and sizes of 0.1-1 ufd can be applied in the power supply , feedback , output capacitors .

some like to add a resistive load in the output which behaves kinda better and often reduces the turn on thump to some point .

Please notice that trying to find what causes the specific problem is not an option since even if you manage and fix it , amplifier of that age will eventually fail again but from another similar cause..better fix it all .